A polyimide resin, which has excellent characteristics in heat resistance and electrical insulation property, is used in various field these days.
The polyimide resin is manufactured from polyamic acid as a raw material. The polyamic acid is produced, in general, by reacting a tetracarboxylic acid dianhydride and diamine under a presence of a solvent. Specifically, adopted for this purpose is, for example, a method of adding in advance the tetracarboxylic acid dianhydride and diamine in a reaction vessel, and then adding a solvent (hereinafter, referred to as a polymerization-use solvent) therein so as to polymerize the tetracarboxylic acid dianhydride and diamine. Thus, in the thus obtained polyamic acid solution, the polymerization-use solvent is utilized as a solvent (hereinafter, referred to as a solution-use solvent) for dissolving polyamic acid.
However, abrupt polymerization reaction takes place before the tetracarboxylic acid dianhydride and diamine are evenly dissolved in the polymerization-use solvent in the above-described method, because the tetracarboxylic acid dianhydride and diamine are added therein in advance. This may cause such a problem in a reaction system of the polymerization that viscosity of part of the reaction system is increased (hereinafter, this problem is referred to as partial viscosity increase). The partial viscosity increase may, in a worst case, result in formation of solid block in the part of the reaction system. As described above, such abrupt polymerization reaction tends to cause “uneven polymerization”, which causes such a problem that every polymerization reaction produces a reaction product (polyamic acid solution) of a different property, whereby the polyimide resin produced from the polyamic acid solution as a raw material has uneven property.
In view of this, various methods have been adopted, for example, a method disclosed in Japanese Publication of Unexamined Patent Application, “Tokukaihei, No. 7-324134” (published on Dec. 12, 1995), Paragraphs [0039] to [0042], in which powder or a solution of the tetracarboxylic acid dianhydride is gradually added into a diamine solution or a diamine suspension, and a method disclosed in Japanese Publication of Unexamined Patent Application, “Tokukaihei, No. 6-336475” (published on Dec. 6, 1994), Paragraph [0025], in which powder or a solution of diamine is gradually added into a solution of the tetracarboxylic acid dianhydride, contrary to the above-mentioned method. Use of those methods makes it possible to avoid the abrupt polymerization reaction. Here, it should be noted that a solvent used for preparing the solution of the tetracarboxylic acid dianhydride and diamine is referred to as a mixing-use solvent.
Incidentally, in most cases, a polyamic acid solution having a high solids content, in other words, a polyamic acid solution having a high concentration of polyamic acid is used in order to form a formed product of a polyimide resin such as a polyimide film. In manufacturing the formed product of the polyimide resin, unnecessary solvent is removed and burned out or is recollected and recycled after imidification of the polyamic acid. Thus, a high solids content and low solvent content in the polyamic acid solution attain low cost in removal/recollection of the solvent, thereby avoiding in an increase in manufacturing cost of the formed product of the polyimide resin.
However, the foregoing prior arts are not only incapable of improving productivity of the polyimide acid, but also unable to suppress the increase in the manufacturing cost of the formed product of the polyimide resin.
Specifically, in the method in which the tetracarboxylic acid dianhydride solution is mixed into diamine for polymerization of the polyamic acid, it is necessary to dissolve the tetracarboxylic acid dianhydride in the mixing-use solvent. On the other hand, in the method in which the powder of the tetracarboxylic acid dianhydride is mixed into the diamine solution, it is necessary to dissolve the tetracarboxylic acid dianhydride in the mixing-use solvent in which the diamine is dissolved. The mixing-use solvent is identical to the polymerization-use solvent, and is in general suitably a non-protic solvent, and especially an amide polarity organic solvent. In short, the tetracarboxylic acid dianhydride should be soluble in the polymerization-use solvent.
However, compounds having poor solubility in the polymerization-use solvent, are recently used as the tetracarboxylic acid dianhydride in many cases, because there is a wide variety of structures of the tetracarboxylic acid dianhydride for use in manufacturing polyamic acid.
Thus, in the method in which diamine is gradually added in the tetracarboxylic acid dianhydride solution, low solubility of the tetracarboxylic acid dianhydride in the mixing-use solvent (polymerization-use solvent) leads to necessity of a large quantity of the mixing-use solvent in order to prepare the tetracarboxylic acid dianhydride solution. This lowers concentration of the tetracarboxylic acid dianhydride in the tetracarboxylic acid dianhydride solution, thus lowering the solids content in the resultant polyamic acid solution. As a result, it becomes difficult to attain the low cost in the removal/recollection of the solvent in manufacturing the formed product of the polyimide resin, as described above.
Because of this, the method in which the powder of the tetracarboxylic acid dianhydride is gradually added in the diamine solution is more popularly applied for the polymerization of the polyamic acid. However, even in this method, it is requisite that the tetracarboxylic acid dianhydride have solubility high enough to be dissolved, without difficulty, (that is, the tetracarboxylic acid dianhydride is sufficiently soluble) in the polymerization-use solvent in which diamine is dissolved. In short, if the tetracarboxylic acid dianhydride has poor solubility in the polymerization-use solvent, the polymerization of the polyamic acid takes longer time, thus lowering the productivity.